html, body {
  padding: 0;
  margin: 0;
  height: 100%;
  width: 100%;
}
.vetical-center {
  position: relative;
  top: 50%; /*偏移*/
  transform: translateY(-50%);
}
.horizontal-center {
  position: relative;
  left: 50%; /*偏移*/
  transform: translateX(-50%);
}
.text-center {
  text-align: center;
}
.bg {
  border: black solid 1px;
}